Coplanar energy transfer

ABSTRACT

An external transmitter inductive coil can be provided in, on, or with a belt designed to be placed externally around a part of a body of a patient. An implantable device (such as a VAD or other medical device) that is implanted within the patient&#39;s body has associated with a receiver inductive coil that gets implanted within that part of the patient&#39;s body along with the device. The externally-located transmitter inductive coil inductively transfers electromagnetic power into that part of the body and thus to the receiver inductive coil. The implanted receiver inductive coil thus wirelessly receives the inductively-transferred electromagnetic power, and operates the implant.

STATEMENT OF RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/588,524, filed Aug. 17, 2012, which claims priority to U.S.Provisional Patent Application Ser. No. 61/525,272, filed Aug. 19, 2011,and U.S. Provisional Patent Application Ser. No. 61/540,140, filed Sep.28, 2011, each of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The invention relates to wireless energy transfer into the body of apatient to power wirelessly a device implanted within the patient'sbody.

BACKGROUND INFORMATION

Wireless power transfer for implanted medical devices is a known andwell-studied subject. The traditional approach is TET (transcutaneousenergy transfer), in which the energy source is directed toward theenergy harvesting device with the goal to minimize RF exposure of thepatient. In one commercial embodiment, the receiver coils are locatedunder the patient's skin and the transmitter above the skin. Such TETsystems are very sensitive to misalignment and movement of the implantedcoil. Additionally, the coil implanted in a separate surgical procedure.Another shortcoming of the current TET solution is that theelectromagnetic field density is so high that it can cause heating ofthe skin and even burns. That is, when the receiver is receiving energy,regular resistance losses within the coil can cause heating to the samevolume of tissue receiving the electromagnetic radiation and add heatingto it. When the transmitter attached to the receiver is transmittingenergy, regular resistance losses within the transmitter coil can causeheating that adds to the receiver regular resistance losses heating andto the receiving electromagnetic radiation heating. The accumulated heatcan become a complex issue. TET systems have also suffered setbacks dueto complexity and lack of efficiency.

Because of the heating issues, there is no TET system commerciallyavailable for use with a ventricular assist device (VAD). In current VADsystems, the power needed for the pump is delivered via an externalpower pack by a transcutaneous power line. The exit site of the driveline from the abdomen provides a portal of entry for pathogens, makingVAD recipients highly vulnerable to device-related infections. Howeverinfectious complications are not limited to VAD systems, as infectionsare common in many medical devices that use transcutaneous power line.

There have been many attempts to develop a superior wireless powertransfer system for use with implanted medical devices. Some knownwireless power transfer approaches are described in U.S. Pat. Nos.6,772,011, 7,741,734, 7,825,543, 7,613,497, 7,825,776, and 7,956,725 andin U.S. Patent Application Publication Nos. 2007-0132587, 2007-0182578,2008-0041930, 2008-0238680, 2009-0243813, 2010-0045114, 2010-0052811,2010-0081379, and 2010-0187913, all of which are incorporated herein byreference in their entireties.

SUMMARY OF THE INVENTION

In general, the invention relates to an external transmitter inductivecoil that can be provided in, on, or with a belt designed to be placedexternally around a part of a body of a patient. This embodimentsuggests new approach for medical implant wireless power transfer, whichwill increase the safety and efficiency, and in parallel reduce thecumbersomeness of traditional TET use by simplify the surgery andplacement process.

The transmitter inductive coil can be one, two, or more turns of anelectrically-conductive material such as a metal wire. The patient canbe a human or an animal, and the part of the body can be the arm, leg,head, or torso of the patient. An implantable device (such as a VAD orother medical device) that is implanted within that part of thepatient's body has associated with it (for example, electrically coupledto it) a receiver inductive coil that gets implanted within that part ofthe patient's body along with the device. The externally-locatedtransmitter inductive coil surrounds at least a portion of the implantedreceiver inductive coil and inductively transfers electromagnetic powerinto that part of the body and thus to the receiver inductive coil. Theimplanted receiver inductive coil thus wirelessly receives theinductively-transferred electromagnetic power from the externaltransmitter coil, and the implanted receiver inductive coil providesthat received power to the implanted implantable device to allow thatdevice to operate. If the device is a VAD, then the power can be used tooperate the pumping action of the VAD.

In one particular aspect, the invention involves a system for wirelesslypowering an implantable device such as a medical device, and the systemcomprises a transmitter inductive coil and a receiver inductive coil.The transmitter inductive coil is configured to be disposed externallyaround a part of a body of a patient (whether human or animal) withinwhich the implantable device is implanted and to inductively transferelectromagnetic power into that part of the body. The receiver inductivecoil is associated with the implantable device and is configured to beimplanted within that part of the patient's body along with theimplantable device to wirelessly receive the inductively-transferredelectromagnetic power and provide that received power to the implantedimplantable device.

Embodiments according to this aspect of the invention can have variousfeatures. For example, the externally-located transmitter inductive coilcan be one, two, or more turns of an electrically-conductive materialsuch as a metal wire, and the external transmitter inductive coil can beprovided in, on, or with a belt designed to be placed externally aroundthe part of the patient's body within which the receiver inductive coilis implanted. For appropriate power transfer from the externaltransmitter inductive coil to the implanted receiver inductive coil, atleast a portion of the implanted receiver inductive coil typically willand should be disposed within an imaginary plane that passes through thepart of the patient's body within which the receiver inductive coil isimplanted and that is defined by the external transmitter inductivecoil. It could be that the entirety of the implanted receiver inductivecoil is disposed with this imaginary plane. Like the externaltransmitter inductive coil, the implantable receiver inductive coil canbe one, two, or more turns of an electrically-conductive material suchas a metal wire. The implantable device can be a blood pump, aventricular assist device (VAD), or some other type of implantablemedical device that requires power to operate, and the part of thepatient's body within which the device/receiver-coil is implanted can bean arm, a leg, a torso, or a head of the patient.

These and other aspects, features, objects, and advantages of theinvention will become apparent through reference to the followingdescription, drawings, and claims. It is noted that aspects of theembodiments described herein are not mutually exclusive and can exist invarious combinations and permutations even if not specifically indicatedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawing(s), like reference characters generally refer to the sameor similar parts throughout the different views. The drawings areintended to illustrate the details of one or more embodiments accordingto the invention and/or the principles of the invention.

FIG. 1 depicts an external belt 102 surrounding an implantable receivercoil 101; the belt can be opened and closed using a buckle 103.

FIG. 2 shows an implantable circuit with a DC-to-DC converter coupled tothe implantable receiver resonance structure 202. The load R₁ can be aVAD or any other power consuming implantable device.

FIG. 3 is a graph showing a relationship between output power and loadresistance.

FIG. 4 shows an alternative circuit coupled to the implantable receivercoil. The circuit has the same resonance structure 202 but uses theinductiveness of the implant VAD 402 and controller 401 as a DC to DC.

FIG. 5 is a circuit that can be used to lock the external transceiverresonance frequency to the implanted receiver resonance frequency.

FIG. 6 is another circuit that can be used to lock the externaltransceiver resonance frequency to the implanted receiver resonancefrequency.

FIG. 7 shows ring coil 701 implanted in the bottom of the pericardiumsack 702.

FIG. 8 shows two ring coil 801 implanted in the bottom of the pulmonarycage.

FIG. 9 shows stent base ring coil 901 located in the descending aorta902.

FIG. 10 shows a model circuit for calculating the efficiency of energytransmission.

FIG. 11 shows a schematic of a series-loaded receiver circuit.

FIG. 12 shows a schematic of a parallel-loaded receiver circuit.

FIG. 13 shows a schematic whereby a simple single-phase rectifyingcircuit is added to a receiver.

FIG. 14 is a flow chart of coarse and fine resonance frequencydetection.

FIG. 15 is a flow chart of coarse and fine frequency base power control.

FIG. 16 shows a configurable capacitor for use with a circuit forlocking the external transceiver resonance.

DESCRIPTION

The invention relates to an external transmitter inductive coil that canbe provided in, on, or with a belt designed to be placed externallyaround a part of a body of a patient. Referring to FIG. 1, a surroundingbelt 102 is depicted with a medical stent 101 therewithin. The stent 101has built into it or incorporated within it a receiver coil of one ormore turns of electrically-conductive material such as copper wire, forexample. The belt 102 has in or on it, around its entire length, one ormore turns of a transmitter coil. Like the receiver coil, thetransmitter coil can have one or more turns of electrically-conductivematerial such as copper wire, for example. Together, the external belt102 with the transmitter coil and the implantable medical device (suchas a stent) with the receiver coil, can be considered a wireless powertransfer system. In use, the transmitter coil is located externallyaround the chest of a patient or around some other part of the patient'sbody such as an arm, a leg, a head, or another part of the patient'storso, and the receiver is implanted within that part of the patient'sbody, such that electromagnetic power inductively transmitted from thesurrounding coil of the belt 102 reaches and is wirelessly received bythe patient-implanted receiver coil from all angles and directions.

This physical arrangement of the external surrounding transmitter coiland the internally implanted receiver coil (that is disposed at leastpartially within an imaginary plane cutting through the patient's bodyand that is formed or defined by the surrounding external transmittercoil) can be referred to as a coplanar arrangement. And the system ofthe external transmitter coil and the implanted receiver coil thus canbe referred to as a coplanar energy transfer (CET) system.

CET is different than a known and common technique referred to astranscutaneous energy transfer (TET). TET only transfers energy throughan area of the skin of a patient to a shallowly-implanted receiver justunder that area of the skin. CET, in sharp contrast, involvessurrounding the implanted receiver coil by placing or wrapping atransmitter coil completely around the part of the patient's body withinwhich the receiver coil is implanted. If the receiver coil is disposedwithin the brain of the patient, for example, then CET involvesdisposing the transmitter coil externally around the corresponding partof the head of the patient such that an imaginary plane defined by thesurrounding transmitter coil extends through at least a portion of thebrain-implanted receiver coil. If the receiver coil is instead implantedwithin the descending aorta of the patient's vasculature, CET involvesdisposing the transmitter coil externally around the corresponding partof the patient's chest such that the imaginary plane defined by thesurrounding transmitter coil extends through at least a portion of theaorta-implanted receiver coil. These are just two examples of where thetransmitter and receiver coils could be located, and other locations arepossible such as the arm or the leg of a patient.

The stent 101 of FIG. 1 has built into it or incorporated within it thereceiver coil, as indicated previously, and in this regard it is notedthat the receiver inductive coil can comprise one or more electricallyconductive fibers or strands that are among the various fibers orstrands that together constitute the stent 101. These fibers or strandsthat comprise the receiver inductive coil can be electrical wires andcan be coated with an electrical insulator. The receiver inductive coilcan be built into or incorporated within the stent 101 in a variety ofother ways.

In one embodiment, the receiver coil is not built into the device withwhich it is associated. In this embodiment, the implantable receivercoil is operatively connected (such as by an electrical wire connection)to the implantable device to be able to provide wirelessly-receivedpower to the implanted device but otherwise could be physically separatefrom and not an integral part of the device itself.

In another embodiment, the receiver coil is built into the device withwhich it is associated.

In one embodiment, the receiver coil is a stent 101 as shown in FIG. 1as the device with which the implantable receiver coil is associated,

In addition to VADs, the receiver coil can be associated with a varietyof other types of implantable devices, including, for example, aconstant glucose meter (CGM), a blood-pressure sensing device, a pulsesensing device, a pacemaker, implantable cardioverter defibrillators(IDC), digital cameras, capsule endoscopies, implanted slow release drugdelivery systems (such as implanted insulin pump) a nerve stimulator, oran implanted ultrasound device.

In operation, the CET system generates lower radio-frequency (RF) energydensities than TET systems. Because CET uses a surrounding externalbelt-like transmitter coil, the RF energy that is inductivelytransmitted into the patient's body from the transmitter coil is spreadout and not concentrated or focused into or onto a particular spot orarea of the patient's body. Using CET, the transmitted energy is spreadout over the external transmitter coil of the CET, resulting intransmitted field strength and power density levels that are lower thanTET systems. Also using a surrounding external belt-like transmittercoil eliminate misalignment problem and reduce dramatically themisplacement problems.

It is noted that a power source must be associated with the externaltransmitter coil to provide that coil with the power that it will thenwirelessly transmit for receipt by the implanted receiver coil. Acontroller unit also typically will be provided to regulate theoperation of the transmitter coil Like the transmitter coil, both thepower source and the controller will be external to the patient. Theexternal source can be an AC current source, and the transmitter coilcan be electrically connected to the AC current source. It also is notedthat the transmitter coil can be a transceiver—that is, capable of bothtransmitting and receiving.

Providing an Optimal Load to the Receiver Resonance Structure:

In one embodiment according to the invention, the device with which theimplantable receiver coil is associated is a ventricular assist device(VAD). In this embodiment a DC-to-DC converter is employed to provide anoptimal load to the receiver inductive coil. The DC-to-DC converter isdesigned to automatically adjust to provide a constant or substantiallyconstant selected optimum load to the receiver inductive coil.Typically, the DC-to-DC converter is implanted within the patient's bodyalong with the receiver inductive coil and the VAD.

FIG. 2 shows a DC-to-DC converter disposed between the receiverinductive coil and the resonance structure 202 (on the left) and a loadR_(L) (on the right). Load 201 may be a VAD or a constant glucose meter,or another implantable device described herein. As shown in FIG. 2, thecircuit can also include a half or full-wave rectification (i.e., usinga diode or diode bridge). As shown in FIG. 2, resonance structure 202 isformed by the receiver inductive coil and a capacitor. However theexternal transmitter inductive coil may also be associated with acapacitor to form a transmitter or transceiver resonance structure.

The optimum load can be determined with reference to FIG. 3 which showsa relationship between power harvesting (W) and an R_(load) value for aparticular circuit, where R_(load) is the internal resistance ofwhatever load is associated with the receiver inductive coil. Whilemerely exemplary, the graph of FIG. 3 shows that the best powerharvesting for the circuit is 80 Ohms or about 80 Ohms. A load with aresistance lower than 80 Ohms will reduce the voltage on the load andthereby reduce the harvested power, and a load with a resistance higherthan 80 Ohms will reduce the current and thereby reduce the harvestedpower. The shape of the curve in the graph of FIG. 3 is determined bythe function (26), provided below. In the case of a VAD, the resistiveload represented by the VAD's motor will change as the mechanical loadon the motor changes, and the depicted circuit (in FIG. 2) with theDC-to-DC converter is what is used to automatically adjust and provide asubstantially constant and optimum load to the receiver inductive coil.

In case of medical implant with high inductive load, like a VAD orimplanted slow release drug delivery system that uses a motor, analternative to the circuit of FIG. 2 is the circuit shown in FIG. 4. Thecircuit of FIG. 4 can be employed to adjust to an ideal working point ofthe receiver inductive coil (or, more accurately, of the receiverresonance structure which, as described above, is the combination of thereceiver coil and its associated capacitor) when the device with whichthe implantable receiver coil is associated is a VAD.

An implant with a brushless DC motor, like a VAD, needs adjustable powercontrol to receive exactly the needed mechanical power. As shown in FIG.3 and by function (26) below and as described above, the best powerharvesting is achieved with the optimum R_(load). In this situation, ahigh quality motor controller, such as MOTION EN Speed Controller SeriesSC 1801 F (Faulhaber GmbH & Co. KG, Schönaich, Germany), can be used asa DC-to-DC converter for adjusting the R_(load) to the optimum valueusing PWM (pulse width modulation). As shown in FIG. 4, a voltage andmotor PWM controller 401 gives full control over the working pointwithout any additional measures.

By controlling the voltage and the DC-to-DC rate, the optimum R_(load)can be achieved. The brushless DC motor of the VAD can be simulated withan equivalent resistor and inductor circuit. The speed of the motor iscontrolled using PWM as the motor input voltage, and the duty cycle isadjusted according to the needed speed. The coils of the VAD's motorflat the current just as is done in DC-to-DC voltage conversion. In thisway, the VAD's motor is used as a DC-to-DC converter, and the reflectedmotor load is dependent on the conversion rate.

Adding a voltage sensor with voltage control adds the capability toselect the voltage in the receiver circuit. This gives full control onthe reflected load (using the PWM mechanism) and on the used power bycontrolling the voltage (using the voltage control). For example, aLPC1102 chip can be used for (NXP Semiconductors N.V., Eindhoven,Netherlands) voltage sensing while an internal PWM engine and cancontrol the voltage by using a transistor like SI8409DB (NXPSemiconductors) for closing the inline from the resonance structure 202.

The voltage control can be done in several ways. One example isharvesting control on/off measured, as shown in FIG. 4, in the implantedreceiver electronics itself. Another example is transmitting powercontrol in the external transmitter/transceiver primary electronics thatcloses the loop according to the V_(sense) in the receiver.

Locking the Receiver and the Transmitter:

Once placed within the body of a patient, the receiver coil shape can bedistorted or modified from its at-rest shape and also can move over timeto some extent as the patient moves, all depending on the particularlocation internally within the patient's body where the receiver coil isplaced. With changing of its shape, the resonance frequency of thereceiver coil changes. It is important for the transceiver resonancestructure to be able to automatically find the receiver's resonance andadjust the transceiver's resonance to that found for the receiver andlock to that found resonance. In other words, the transceiver must havethe capability to detect the receiver's resonance frequency and thenlock to that detected receiver's resonance frequency.

As described in FIG. 14 and in FIG. 15 the transceiver can detect thereceiver's resonance frequency in two phases. First, in a coarse phase,when no pre-detected frequency is available, the transceiver uses a fastfrequency detection process to roughly detect the receiver resonancefrequency or else just start at some predetermined frequency. Second, ina fine phase that occurs after the coarse phase, the transceiver uses anongoing process of fine tuning to detect the receiver resonancefrequency.

The main difference between the two procedures is the simplicity of thesolution. FIG. 15 describes a very simple system where the coarse phasedetects roughly the resonance, which then becomes the minimum frequencylimit. (The system of FIG. 15 doesn't use the resonance frequencyexactly, it uses a frequency above (or below) the resonance and thencontrols the transfer power by tuning the frequency). This is a simplesystem and it can work in strong coupling environment like the CETsystem. In other instances, when the coupling is lower due to distanceor receiver/transceiver size/quality it is necessary to use the exactresonance frequency to be able to transfer the needed power.

FIG. 14 describes the fine process that occurs after the first coarseadjust approximately determines the transmitter resonance. In the coarsephase, a microcontroller (MCU) associated with the transceiver resonancestructure can have preliminary information about the receiver resonancefrequency. The MCU will change the transceiver's driver frequency oneafter the other and detect the root mean square (RMS) current in the oneor more coils of the transceiver. At the end of this phase, the MCU hasthe result of the entire frequency spectrum, and it can automaticallyselect (as a result of its software programming) the best first coarsefrequency, F_(coarse).

After the coarse phase, the fine phase begins, in which the MCU'ssoftware programming dictates the selected frequency from the coarsephase as the best known resonance, F_(best). Once in the fine phase, theMCU stores the RMS current, adds single F_(delta) to the previousfrequency and stores that RMS current. By comparing these two RMScurrents, the transceiver's MCU determines whether to add F_(delta) orto reduce F_(delta) from the previous F_(best). The equation used is asfollows: F_(best)=F_(best)+/−F_(delta). Then, the tranceiver's resonancefrequency is locked to the receiver's resonance frequency until the nextfine phase process occurs. The fine phase process can occur periodicallyevery T_(fine).

Locking the transceiver resonance to the detected receiver resonanceinvolves the transceiver coil automatically adjusting its capacitors,which can be accomplished using either the circuit shown in FIG. 5, orthe circuit shown in FIG. 6, each of which is a resonance LC (inductanceand capacitance) structure.

In the circuit of FIG. 5, the MCU forces a fix frequency by generatingP_(out) pulse in the requested frequency and push the driver circuit. Inso doing, the MCU thereby calibrates the configurable capacitor to getresonance. The MCU receives the feedback phase, and adjusts it to theresonance. In resonance, the feedback phase P_(in) should be exact asthe generated one P_(out). The MCU then compares the output P_(out) tothe input P_(in) to validate the resonance. The MCU should adjust thecapacitors according to the phase until P_(in)=P_(out)

In the circuit of FIG. 6, the circuit is a self-oscillating circuit, andthus is always in resonance, however the MCU can adjust the frequency bychanging the capacitors. The MCU can add capacitors to the capacitorsarray or remove capacitors as described in FIG. 16.

Although FIGS. 5 and 6 show two particular circuits that can be used, itis noted that a variety of variants of phase locked loop (PLL)algorithms and implementing circuits can be used to compensate forimpedance changes of the coils by adjusting capacitor value.

Placement in a Patient's Body of the Receiver Resonance Structure:

The receiver inductive coil can be placed within the body of a patientat a variety of internal locations. FIGS. 7, 8, and 9 illustrate threeparticular examples of a placement location inside the body of apatient.

As shown in FIG. 7, the receiver coil 701 may be placed in the base ofthe flat part of the pericardia 702, which surrounds the heart 704. Themain added value in placing the receiver coil 701 in the pericardia 702with a VAD is that the pericardia 702 is relatively flat and open intypical VAD surgery. The receiver coil 701 can be glued to thepericardia 702 boundaries, e.g., with surgical glue.

In FIG. 8, it is shown that the receiver coil 801 of a VAD can be placedin the pulmonary cage. One advantage of placing the coil 801 in thepulmonary cage is that the VAD will not disturb the magnetic powerharvesting, and that pulmonary cage is relatively easy to access duringthe VAD surgery.

As shown in FIG. 9, the receiver coil 901 may also be placed in anartery 902. The Aorta or the Vena Cava are particularly well-suited forplacement of the receiver coil 901 because each is oriented verticallywith respect to a plane that cuts in a cross section through the torsoof the patient. Placement of the receiver coil 901 in the Aorta or theVena Cava also allows the receiver coil 901 to be associated with animplantable stent.

Further Disclosure Related to Providing an Optimum Load to the ReceiverResonance Structure:

Having presented various details of various embodiments according to theinvention, some theory, equations, and calculations relevant toproviding an optimum load to the receiver resonance structure will nowbe presented.

The ratio between the distance D from the transmitting coil to receivingcoil and the wavelength λ is as follows:

$\begin{matrix}{{\frac{D}{\lambda} = \frac{Df}{c}},} & (1)\end{matrix}$where f is the transmitting frequency and c=3·10⁸ m/s is the speed oflight.

Given that the maximum distance D_(max) does not exceed 0.4 m and theworking frequency is f=100 kHz, the ratio D_(max)/λ=0.00013<<1. Thus, wecan conclude that the receiving coil is in the quasi-static area, and wecan neglect the effects of the phase difference due to the wavepropagation.

The amplitude of the voltage induced in the receiving coil according tothe Faraday's law [1] is as follows:

$\begin{matrix}{{{v_{r}(t)} = {{- \frac{d\;\Phi}{d\; t}} = {{- \frac{d}{d\; t}}\left( {B \cdot a} \right)}}},} & (2)\end{matrix}$where Φ is the magnetic flux through the receiving coil, B is themagnetic flux density, and a is the effective area of the receivingcoil.

To estimate the maximum induced voltage (2), assume that the receivingcoil is located coaxially with the transmitting coil at its center,where the magnetic flux density B can be calculated as follows [1]:

$\begin{matrix}{{B = {\frac{\mu_{r}\mu_{0}I_{t}N_{t}}{2\; R_{t}}{\sin\left( {2\;\pi\; f\; t} \right)}}},} & (3)\end{matrix}$where μ_(r) is the relative permeability of media, μ₀=4π10⁷ V·s/(A·m) isthe permeability of vacuum, I_(t) is the amplitude of the current in thetransmitting coil, and R_(t) and N_(t) are the radius and number ofturns of the transmitting coil correspondingly.

The effective area of the receiving coil can be calculated as follows:a=πR_(r) ²N_(r),  (4)where R_(r) and N_(r) are the radius and the number of turns of thereceiving coil correspondingly.

Substituting (3) and (4) into (2) and differentiating with respect tothe time, gives the following expression for the amplitude of thevoltage induced in the receiving coil:

$\begin{matrix}{V_{r} = {2\pi\; f\frac{\mu_{r}\mu_{0}I_{t}N_{t}}{2R_{t}}{\pi R}_{r}^{2}{N_{r}.}}} & (5)\end{matrix}$

The transmitting and the receiving coils can be seen as two coupledinductors, as follows:

$\begin{matrix}\left\{ {\begin{matrix}{v_{t} = {{L_{t}\frac{d\; i_{t}}{d\; t}} - {M\frac{d\; i_{r}}{d\; t}}}} \\{v_{r} = {{{- M}\frac{d\; i_{t}}{d\; t}} + {L_{r}\frac{d\; i_{r}}{d\; t}}}}\end{matrix},} \right. & (6)\end{matrix}$where v_(t) and v_(r) are the transmitter and receiver coils voltages,i_(t) and i_(r) their currents, and M is the mutual inductance.

Assuming that the current in both coils is a sine-wave of frequencyω=2πf, (6) can be written as follows:

$\begin{matrix}\left\{ {\begin{matrix}{v_{t} = {{j\;\omega\; L_{t}i_{t}} - {j\;\omega\;{Mi}_{r}}}} \\{v_{r} = {{{- j}\;\omega\;{Mi}_{t}} + {j\;\omega\; L_{r}i_{r}}}}\end{matrix}.} \right. & (7)\end{matrix}$

The mutual inductance M can be found from the open circuit experiment,where i_(r)=0:

$\begin{matrix}\left\{ {\begin{matrix}{{v_{t}}_{i_{r} = 0} = {j\;\omega\; L_{t}i_{t}}} \\{{v_{r}}_{i_{r} = 0} = {{- j}\;\omega\;{Mi}_{t}}}\end{matrix}.} \right. & (8)\end{matrix}$

Rearranging the second equation of (8) with respect to M andsubstituting (2)-(5) gives us:

$\begin{matrix}{\left. M \right|_{i_{r} = 0} = {{- \frac{v_{r}}{j\;\omega\; i_{t}}} = {{- \frac{{- \frac{d}{d\; t}}\left( {B \cdot a} \right)}{j\;\omega\; i_{t}}} = {{\frac{\mu_{r}\mu_{0}N_{t}}{2R_{t}}\pi\; R_{r}^{2}N_{r}} = {3.9\mspace{14mu}\mu\;{H.}}}}}} & (9)\end{matrix}$

The value of M obtained in (9) increases as a function of the relativepermeability μ_(r) of the receiver core.

For the purpose of efficiency calculation, assume that the transmittercoil is loaded with a series resonant capacitor and the receiver coil isloaded to form a series resonant circuit as describe in FIG. 10.

The transmitter current is calculated using the coupled-inductor model(7), as follows:

$\begin{matrix}{{i_{t} = \frac{v_{s}}{R_{t} + \frac{\left( {\omega\; M} \right)^{2}}{R_{r} + R_{L}}}},} & (24)\end{matrix}$where v_(s)=2V_(DD)/π is the effective voltage of the source V_(dr) atthe first harmonic of the excitation frequency, R_(t) is the activeresistance of the transmitter coil, R_(r) the active resistance of thereceiver coil, and R_(L) is the load resistance.

The amplitude of the load voltage is given by:

$\begin{matrix}{{V_{L} = {{\frac{2\omega\;{MV}_{DD}\text{/}\pi}{R_{t} + \frac{\left( {\omega\; M} \right)^{2}}{R_{r} + R_{L}}} \cdot \frac{R_{L}}{R_{r} + R_{L}}} = {\frac{2\omega\;{MV}_{DD}\text{/}\pi}{{R_{t}\left( {R_{r} + R_{L}} \right)} + \left( {\omega\; M} \right)^{2}} \cdot R_{L}}}},} & (25)\end{matrix}$where V_(DD) is the supply voltage of the half-bridge driver of thetransmitter.

From here, the load power is given by:

$\begin{matrix}{P_{L} = {\frac{V_{L}^{2}}{2R_{L}} = {\frac{2\left( {\omega\;{MV}_{DD}\text{/}\pi} \right)^{2}R_{L}}{\left( {{R_{t}\left( {R_{r} + R_{L}} \right)} + \left( {\omega\; M} \right)^{2}} \right)^{2}}.}}} & (26)\end{matrix}$

Differentiating (26) with respect to R_(L) gives the load resistancethat maximizes the load power, as follows:

$\begin{matrix}{R_{Lopt} = {R_{r} + {\frac{\left( {\omega\; M} \right)^{2}}{R_{t}}.}}} & (27)\end{matrix}$

Substituting (27) into (26) yields:

$\begin{matrix}{P_{Lopt} = {0.5{\frac{\left( {V_{DD}\text{/}\pi} \right)^{2}{\left( {\omega\; M} \right)^{2}/R_{t}}}{{R_{t}R_{r}} + \left( {\omega\; M} \right)^{2}}.}}} & (28)\end{matrix}$

Rearranging (28) with respect to the driver voltage gives:

$\begin{matrix}{V_{DD} = {\frac{\pi}{\omega\; M}{\sqrt{2P_{Lopt}{R_{t}\left( {{R_{t}R_{r}} + \left( {\omega\; M} \right)^{2}} \right)}}.}}} & (29)\end{matrix}$

The input power is:

$\begin{matrix}{{P_{t} = {{\frac{V_{DD}}{2\pi}{\int_{0}^{\pi\text{/}\omega}{{i_{t}(t)}\ d\; t}}} = {2\left( \frac{V_{DD}}{\pi} \right)^{2}\frac{R_{r} + R_{L}}{{R_{t}\left( {R_{r} + R_{L}} \right)} + \left( {\omega\; M} \right)^{2}}}}},} & (30)\end{matrix}$while its optimal value considering (27) is:

$\begin{matrix}{P_{topt} = {\left( \frac{V_{DD}}{\pi} \right)^{2}{\frac{{2R_{r}} + {\left( {\omega\; M} \right)^{2}\text{/}R_{t}}}{{R_{t}R_{r}} + \left( {\omega\; M} \right)^{2}}.}}} & (31)\end{matrix}$

Dividing (29) by (31) gives the efficiency of the wireless powertransmission corresponding to the optimum load resistance:

$\begin{matrix}{\eta_{opt} = {\frac{P_{Lopt}}{P_{topt}} = {0.5{\frac{1}{1 + \frac{2R_{r}R_{t}}{\left( {\omega\; M} \right)^{2}}}.}}}} & (32)\end{matrix}$

The general expression for the efficiency is:

$\begin{matrix}{\eta = {\frac{P_{L}}{P_{t}} = {\frac{\left( {\omega\; M} \right)^{2}}{{R_{t}\left( {R_{r} + R_{L}} \right)} + \left( {\omega\; M} \right)^{2}} \cdot {\frac{R_{L}}{R_{r} + R_{L}}.}}}} & (33)\end{matrix}$

Differentiating (33) with respect to R_(L) gives the load resistancethat maximizes the efficiency:

$\begin{matrix}{R_{L\;\eta\;\max} = {\sqrt{R_{r}^{2} + \frac{\left( {\omega\; M} \right)^{2}R_{r}}{R_{t}}}.}} & (34)\end{matrix}$

The maximum efficiency can be calculated by substituting (34) into (33).

$\begin{matrix}{\eta_{\max} = {0.5{\frac{1}{1 + \frac{2R_{r}R_{t}}{\left( {\omega\; M} \right)^{2}}}.}}} & (32)\end{matrix}$

The maximum efficiency and maximum load power for the parallel-loadedreceiver is identical to that of the series one. The optimal loadresistance and maximizing the efficiency for the parallel-loadedreceiver differ from (27) and (34). However, the derivation is similar.The specific formulae for the load resistance is not developed here andinstead we find the optimal resistance using computer simulations toollike PSPICE® (Cadence Design Systems, San Jose, Calif.), afull-featured, native analog and mixed-signal circuit simulation tool.

The circuit shown in FIG. 11 is a series-loaded receiver. The source Vdris built from two BUZ11 N-MOSFETs driven by the IR2111 gate driver. The0.1 Ohm resistor is used for the transmitter current monitoring. Boththe transmitter and receiver capacitors are chosen with low ESR. Theload resistance is chosen as R_(L)=0.5 Ohm, and the driver voltageV_(DD)=12 V. Substituting these values and the other setup parameters(R_(t)=1 Ohm, R_(r)=0.65 Ohm, M=2.056 μH) into (26) gives for P_(L)=3.16W. The measured voltage amplitude on the load resistance is 1.75 V,which corresponds to P_(L)=3.1 W. The input power drawn from the powersupply is P_(in)=V_(DD)/π·I=12/3.14·2.8=10.7 W. The efficiency isη=P_(L)/P_(in)=28%. It is noted that the load resistance is notoptimized for the maximum output power.

The circuit shown in FIG. 12 is a parallel-loaded receiver. The sourceV2 is built from two BUZ11 N-MOSFETs driven by the IR2111 gate driver.The 0.1 Ohm resistor is used for the current monitoring. Both thetransmitter and receiver capacitors are chosen with low ESR.Substituting the model parameters into (29) gives V_(DD)=11.5 V forP_(L)=5 W. Computer simulations have shown that the maximum load powerof 4.85 W is obtained for R_(L)=80 Ohm. This result closely correlateswith laboratory measurements, where an output power of 4.5 W wasmeasured for V_(DD)=12 V. The input power drawn from the power supply isP_(in)=V_(DD)/π·I=12/3.14·4.2=16.05 W. The efficiency isη₇=P_(L)/P_(in)=28%.

Inserting a simple single-phase rectifying circuit before R4, as shownin the circuit in FIG. 13, takes about 0.2 W dissipated on the diodewith 2 A peak diode current and 44 V peak diode reverse voltage. Thepeak voltage on the receiver capacitor is 25 V, and the peak voltage onthe transmitter capacitor is 500 V.

Various modifications may be made to the embodiments disclosed herein.The disclosed embodiments and details should not be construed aslimiting but instead as illustrative of some embodiments and of theprinciples of the invention.

INCORPORATION BY REFERENCE

References and citations to other documents, such as patents, patentapplications, patent publications, journals, books, papers, webcontents, have been made throughout this disclosure. All such documentsare hereby incorporated herein by reference in their entirety for allpurposes.

EQUIVALENTS

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The foregoingembodiments are therefore to be considered in all respects illustrativerather than limiting on the invention described herein. Scope of theinvention is thus indicated by the appended claims rather than by theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

The invention claimed is:
 1. A system for wirelessly powering aventricular assist device (VAD) implanted within a body of a patient,comprising: a receiver inductive coil associated with the VAD andconfigured to be disposed in the pleural cavity of the patient's body towirelessly receive inductively-transferred electromagnetic power andprovide that received power to the VAD; and a transmitter inductive coilcomprising one or more turns of electrically-conductive material thatare configured to surround the pleural cavity by being disposedexternally and completely around a part of the patient's body withinwhich the pleural cavity is located, the transmitter inductive coilconfigured to inductively transfer electromagnetic power to the receiverinductive coil.
 2. The system of claim 1 wherein at least one capacitoris associated with the transmitter inductive coil to form a transceiverresonance structure.
 3. The system of claim 1 wherein at least onecapacitor is associated with the receiver inductive coil to form areceiver resonance structure.
 4. The system of claim 1 wherein thetransmitter inductive coil comprises a transceiver capable of bothtransmitting and receiving.
 5. The system of claim 1 wherein thereceiver inductive coil uses a coil of the VAD to adjust to an idealworking point of the receiver inductive coil.
 6. The system of claim 1further comprising a second receiver inductive coil and wherein the tworeceiver inductive coils are coupled in series and function together asa single receiver inductive coil.
 7. The system of claim 1 furthercomprising a circuit associated with the receiver inductive coil andwherein the circuit detects the resonance frequency of the receiverinductive coil and locks the resonance frequency of the transmitterinductive coil to the resonance frequency of the receiver inductivecoil.
 8. The system of claim 1 further comprising a circuit associatedwith the transmitter inductive coil and wherein the circuit compensatesand keeps the resonance frequency of the transmitter inductive coillocked to the resonance frequency of the receiver inductive coil whenthe shape and thus the impedance of the transmitter inductive coilchanges.
 9. The system of claim 1 wherein a DC-to-DC converterassociated with the VAD automatically adjusts to provide a substantiallyconstant and optimum load to the receiver inductive coils.
 10. Thesystem of claim 9 wherein the DC-to-DC converter is located between amotor of the VAD and the receiver inductive coil.
 11. A system forwirelessly powering a ventricular assist device (VAD) implanted within abody of a patient, comprising: a receiver inductive coil associated withthe VAD and configured to be disposed in a left pleural cavity of thepatient's body to wirelessly receive inductively-transferredelectromagnetic power and provide that received power to the VAD; asecond receiver inductive coil associated with the VAD and configured tobe disposed in a right pleural cavity of the patient's body towirelessly receive the inductively-transferred electromagnetic power andprovide that received power to the VAD; and a transmitter inductive coilcomprising one or more turns of electrically-conductive material thatare configured to surround the left and right pleural cavities by beingdisposed externally and completely around a part of the patient's bodywithin which the left and right pleural cavities are located, thetransmitter inductive coil configured to inductively transferelectromagnetic power to the receiver inductive coils.
 12. The system ofclaim 11 wherein at least one capacitor is associated with the receiverinductive coils to form a receiver resonance structure.
 13. The systemof claim 12 wherein at least one capacitor is associated with each ofthe receiver inductive coils to form receiver resonance structures. 14.The system of claim 11 wherein the receiver inductive coils use a coilof the VAD to adjust to an ideal working point of the receiver inductivecoils.
 15. The system of claim 14 wherein each of the receiver inductivecoils uses a coil of the VAD to adjust to an ideal working point of arespective receiver inductive coil.
 16. The system of claim 11 furthercomprising a circuit associated with the receiver inductive coils andwherein the circuit detects the resonance frequency of the receiverinductive coils and locks the resonance frequency of the transmitterinductive coil to the resonance frequency of the receiver inductivecoils.
 17. The system of claim 11 wherein a DC-to-DC converterassociated with the VAD automatically adjusts to provide a substantiallyconstant and optimum load to the receiver inductive coils.
 18. Thesystem of claim 11 wherein at least one capacitor is associated with thetransmitter inductive coil to form a transceiver resonance structure.19. The system of claim 11 wherein the transmitter inductive coilcomprises a transceiver capable of both transmitting and receiving. 20.The system of claim 11 further comprising a circuit associated with thetransmitter inductive coil and wherein the circuit compensates and keepsthe resonance frequency of the transmitter inductive coil locked to theresonance frequency of the receiver inductive coils when the shape andthus the impedance of the transmitter inductive coil changes.